1. Field of Invention
The invention is related to a microwave transmission system comprising a microwave transmitter coupled to at least one microwave receiver via a transmission link, an input of the microwave receiver being coupled to a first input of a mixer via a transmission line, a local oscillator being coupled to a second input of the mixer. The invention is also related to a receiver and a frequency conversion unit for use in such a transmission system.
2. Description of the Related Art
A transmission system of the aforementioned type is known from the book "Microwave devices" by M. J. Howes and D. V. Morgan, published by Wiley Interscience, ISBN 0 471 41729 7. pp. 344-345.
Recently two new frequency bands in the microwave range have been allocated for broadcast purposes. These are the bands from 40.5 GHz-42.5 GHz in CEPT (Conference of European Post and Telecommunication Administrations) countries and from 27.5 GHz-29.5 GHz in other countries. There are several plans to use these frequency bands for distributing TV programs direct to the homes of the subscribers. Also other microwave frequencies can be used if they are made available for broadcast purposes by the regulatory authorities.
In the prior art system a microwave signal is transmitted by a transmitter, which microwave signal is received by a microwave receiver. The microwave receiver comprises a mixer having one input coupled to an antenna, and having one input coupled to a local oscillator. The output of the mixer carrying an IF signal having a frequency of 750 MHz is coupled via a filter to a demodulator, for demodulating the received signal. With widespread use of such transmission systems, care has to be taken to prevent the generation of interference by such systems. One possible source of interference is the radiation of the local oscillator signal. This local oscillator signal leaks via the mixer to the antenna where it is transmitted into free space. In the prior art system a complex bandpass filter would be required to prevent this undesired radiation.